1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to wireless communication using a relay technology.
2. Related Art
A wireless local area network (LAN) technology has been prominent as a wireless communication technology providing a high speed data service in an unlicensed band. Unlike an existing cellular system, in the wireless LAN technology, since an access point (AP) serving as a base station is operated when a power supply thereof is connected to a wired network, the access point may be easily installed by anyone and data communication may be performed at a low cost, such that the wireless LAN technology has been generalized.
The wireless LAN technology has an advantage in which a decentralized operation is simple and has also been spread to a sensor network and a smart utility network. In the case of the sensor network and the smart utility network, since an amount of data to be transmitted is not much and a transmission period of the data is also short, it is more important to expand a service coverage than to increase a transmission data rate.
Most of wireless LAN transmission specifications have been developed in a form in which a multi-antenna is used in order to increase a transmission rate or a bandwidth is expanded. As typical examples thereof, there are institute of electrical and electronics Engineers (IEEE) 802.11n and IEEE 802.11ac. However, in a general network environment, the increase in a transmission rate as described above is significantly important in view of an increase in network capacity; however, in a network for collecting information such as the sensor network, it is more important to expand a coverage. In the case in which the coverage is expanded, since information may be collected in a wide area through a small number of APs, the sensor network may be build at a low cost.
As coverage increasing schemes adopted in existing communication systems, a channel coding scheme having a low code rate, a repetition scheme in which decoding may be performed in a low signal-to-noise ratio, or a beamforming scheme of allowing a high power signal to arrive at a desired receive end may be used. However, in these schemes, a physical layer specification is significantly modified or a cost burden problem due to use of a plurality of antennas is caused. Therefore, a technology of increasing a coverage in a wireless LAN system, particularly, a technology of increasing a service area of a wireless LAN using a relay technology has been demanded.
A relay scheme means a scheme of relaying and transmitting data received from a transmit end to a receive end using an idle terminal between the transmit end and the receive end as a relay terminal A system such as the 3rd generation partnership project (3GPP) long term evolution (LTE) and IEEE 802.16 adopts a relay scheme in a form in which a basestation performs all controls by changing a frame structure, thereby making it possible to obtain an effect such as expansion of a coverage, or the like. However, in the case of the wireless LAN, since there is no base station performing a centralized control function, after a relay link is set in a handshaking scheme of the wireless LAN, relay transmission starts. Since an object of a scheme of adding a relay function in the wireless LAN is to improve a transmission rate rather than to expand a coverage, a handshaking protocol that may be used only in the case in which both of a relay end and a receive end are present in a transmission coverage of a transmit end has been defined.
Therefore, there is a need for a method for transmitting data to a receive end positioned outside a transmission coverage of a transmit end.